Left: An illustration of the nozzle delivering xenon atoms onto the device. Middle: a close-up image of the bridgelike resonator. Right: An illustration of the atoms sticking, unsticking, and sliding off the device surface.

Bring In the (Nano) Noise

At the forefront of nanotechnology, researchers design miniature
machines to do big jobs, from treating diseases to harnessing
sunlight for energy. But as they push the limits of this
technology, devices are becoming so small and sensitive that the
behavior of individual atoms starts to get in the way. Now Caltech
researchers have, for the first time, measured and characterized
these atomic fluctuations—which cause statistical
noise—in a nanoscale device.

Physicist Michael Roukes and his colleagues specialize in
building devices called nanoelectromechanical systems—NEMS
for short—which have a myriad of applications. For example,
by detecting the presence of proteins that are markers of disease,
the devices can serve as cheap and portable diagnostic
tools—useful for keeping people healthy in poor and rural
parts of the world. Similar gadgets can measure toxic gases in an
enclosed room, providing a warning for the inhabitants.

Two years ago, Roukes's group created the world's first
nanomechanical mass spectrometer, enabling the researchers to
measure the mass of a single biological molecule. The device, a
resonator that resembles a tiny bridge, consists of a thin strip of
material 2 microns long and 100 nanometers wide that vibrates at a
resonant frequency of several hundred megahertz. When an atom is
placed on the bridge, the frequency shifts in proportion to the
atom's mass.

But with increasingly sensitive devices, the random motions of the
atoms come into play, generating statistical noise. "It's like fog
or smoke that obscures what you're trying to measure," says Roukes,
who's a professor of physics, applied physics, and bioengineering.
In order to distinguish signal from noise, researchers have to
understand what's causing the ruckus.

So Roukes—along with former graduate student and staff
scientist Philip X. L. Feng, former graduate student Ya-Tang (Jack)
Yang, and former postdoc Carlo Callegari—set out to measure
this noise in a NEMS resonator. They described their results in the
April issue of the journal Nano
Letters.

In their experiment, the researchers sprayed xenon gas onto a
bridgelike resonator that's similar to the one they used to weigh
biological molecules. The xenon can accumulate in a one-atom-thick
layer on the surface, like marbles covering a table. In such an
arrangement—a so-called monolayer—the atoms are packed
so tightly together that they don't have much room to move. But to
study noise, the researchers created a submonolayer, which doesn't
have enough atoms to completely cover the surface of the resonator.
Because of the extra space, the atoms have more freedom to move
around, which generates more noise in the system.

The atoms in the submonolayer do one of three things: they stick to
the surface, become unstuck and fly off, or slide off. Or in
physics speak, the atoms adsorb, desorb, or diffuse. Previous
theories had predicted that the noise is most likely due to atoms
sticking and unsticking. But now that the researchers were able to
observe what actually happens in such a device, they discovered
that diffusion dominates the noise. What's noteworthy, the
researchers say, is that they found that when an atom slides along
the surface of the resonator, it causes the device's vibrating
frequency to fluctuate. This is the first time anyone has measured
this effect, since previous devices were not sensitive to this sort
of diffusion. They also found new power laws in the spectra of
noise frequencies—quantitative descriptions of the
frequencies at which the atoms vibrate.

There's still a lot more to learn about the physics of this noise,
the researchers say. Ultimately, they will need to figure out how
to get rid of it or suppress it to build better NEMS devices. But
understanding this noise—by measuring the random movement of
individual atoms—is itself fascinating science, Roukes says.
"It's a new window into how things work in the nanoscale
world."